1. Field of the Invention
The present invention relates to an air-fuel ratio sensor, to a structure for mounting the sensor to an exhaust system component of a motorcycle, and to an exhaust subassembly incorporating the sensor. More particularly, the present invention relates to a mounting structure for affixing an air-fuel ratio sensor to an exhaust pipe in a motorcycle, and to a related exhaust subassembly including the sensor, the mounting structure, and the exhaust pipe.
2. Background Art
It is well known to employ an oxygen sensor in a motorcycle exhaust system, so as to improve the catalytic efficiency thereof. In a motorcycle having a multi-cylinder engine, such an oxygen sensor is typically mounted on a convergent connector of exhaust pipes at a position upstream of a catalytic converter and spaced apart from the engine on the rear lower side thereof. The catalytic converter is used for exhaust purification, for example, and may comprise a three-way catalyst. In a motorcycle having a single-cylinder engine, such an oxygen sensor is located in an empty space, the space selected such that the sensor does not interfere with the engine body. This position improves the maintainability of the sensor and protects the sensor.
Related to this mounting structure, there has been proposed an invention entitled “Air-Fuel Ratio Control Device for Internal Combustion Engine” (Japanese Laid-open Patent No. Sho 59-74360). In Japanese Laid-open Patent No. Sho 59-74360, a catalyst for exhaust purification is provided in an exhaust pipe of an internal combustion engine for a vehicle, and an exhaust gas sensor is mounted upstream of the catalyst. The composition of an exhaust gas to be supplied to the catalyst is detected by the exhaust gas sensor, and the air-fuel ratio for an air-fuel mixture to be taken into the engine is controlled according to a detection signal output from the exhaust gas sensor. According to the description in Japanese Laid-open Patent No. Sho 59-74360, feedback control of such an air-fuel ratio is performed by a known air-fuel ratio control device, so that the composition of the exhaust gas is optimized, allowing best purification of the exhaust gas by the catalyst. Further, Japanese Laid-open Patent No. Sho 59-74360 also discloses that an oxygen sensor for detecting the concentration of oxygen in an exhaust gas is generally used as the exhaust gas sensor.
Further, according to the description in an invention entitled “Exhaust Gas Sensor Device in Motorcycle” (Japanese Laid-open Patent No. 2000-335467), an exhaust gas sensor, for detecting the concentration of oxygen or the like in an exhaust gas discharged from an engine mounted on a motorcycle, may be mounted on an upper wall of an exhaust pipe at a position on the front side of a right projecting portion of a crankcase, and on the outer side of a right side surface of a cylinder block.
In both of the references cited above, an oxygen sensor is used as the exhaust gas sensor. This kind of oxygen sensor determines whether the air-fuel ratio is rich or lean with respect to a stoichiometric air-fuel ratio in a narrow region near the stoichiometric air-fuel ratio, but it is not suitable for linear detection of the oxygen concentration over a wide range of values.
Also in a motorcycle, it is desirable to detect the air-fuel ratio in a wide region by using an air-fuel ratio sensor in place of the oxygen sensor mentioned above. In other words, the oxygen sensor merely detects the presence or absence of oxygen in an exhaust gas as mentioned above, whereas the air-fuel ratio sensor can linearly detect the oxygen concentration in an exhaust gas over a wide range by using an element whose output voltage changes in proportion to the oxygen concentration. However, since the air-fuel ratio sensor detects the oxygen concentration over a wide range rather than in a narrow region near the stoichiometric air-fuel ratio as mentioned above, the detection accuracy of the air-fuel ratio sensor may be affected significantly by the location of the air-fuel ratio sensor.
Accordingly, it is an object of the present invention to provide a mounting structure for an air-fuel ratio sensor on a motorcycle, wherein the concentration of oxygen or the like can be efficiently and accurately detected by the air-fuel ratio sensor.